This is a very important subject. A good understanding will greatly increase the likelihood that your job will meet your expectations.
RGB and CMYK are known as ‘colour spaces’. RGB stands for Red, Green and Blue. CMYK stands for Cyan, Magenta, Yellow and Black.
RGB is the ‘colour space’ used by computer monitors and digital cameras. All conventional (offset) and digital printing presses use CMYK.
Different colour gamuts
The problem is that RGB and CMYK have different colour ‘gamuts’. A colour gamut is the range of colours that a colour space can reproduce. RGB has a wider colour gamut than CMYK. That is, RGB is capable of reproducing more colours than CMYK. RGB tends to be able to reproduce colours at the extremes of red, green and blue which CMYK is incapable of reproducing. In other words, bright reds, greens and blues will not be quite so bright when converted to CMYK. You will, for instance, see this in particular when converting a deep cobalt blue sky to CMYK.
It is highly likely that the images and background tints you work with whilst preparing your job will use the RGB colour space. At some stage they will need to be converted to CMYK.
Printers use CMYK
The raster image processors (RIPs) that all presses use to prepare files for printing have RGB to CMYK conversion ‘algorithms’ built into them. The results can be less than satisfactory, and we don’t have a great deal of control over the process. Therefore if we receive files contain elements in the RGB colour space, we convert them prior to sending files to the RIP using a standard we have found to be predictable and generally satisfactory.
However it is far better if you do the conversion. You will see the result for yourself. In many cases, you won’t see any difference, but if you are not happy with the result, you can do something about it. You can, for instance, open the image in Adobe PhotoShop and make adjustments that may enable you to recover some of the brightness lost in the conversion process. You might also experiment with different conversion profiles to see which provides the best result.
Monitors display RGB
Even though computer monitors display colours in the RGB colour space, your job will look more like the printed job if you convert it to CMYK and then view it on your monitor. We recommend you carry out the conversion process early in the process of preparing your job.
So please, for the best results, make sure your job has been converted from RGB to CMYK before sending it to us.
Please send us composite CMYK files (one PDF file), not pre-separated files (four PDF files).
If you can’t work out how to work in the CMYK colour space in your software, please phone or email us for advice. Click here for our contact details.
If colours are absolutely critical, then please consider asking for a hard copy proof. We will print two copies, and retain one so we can match it when we print your job. It will usually be printed on the press your job will be printed on, and on the paper you have requested.
Another technical explanation
Here is how an article that appeared in a US printing news site describes the problem.
Graphic designers and photographers know the pain of translating RGB image information on screen into CMYK print on paper. Even with the best understanding of the alchemy of color management, RGB displays and CMYK printing processes simply cannot recreate the same hues and intensities of every color.
Most displays use the additive red-green-blue color model, which attempts to mimic how our eyes interpret color. By mixing various intensities of red, green, and blue light in each pixel location, displays can reproduce some 16.7 million combinations of hue and intensity. Even the best displays, however, can only reproduce a limited subset of the colors most humans can perceive.
Printing processes, on the other hand, use a subtractive cyan-magenta-yellow color model. By increasing the amount of cyan, magenta, or yellow pigments or dyes, wavelengths are gradually subtracted from white light to produce a range of colors. Photographic prints use CMY dyes, whereas many inkjets and most commercial printing processes add black ink (‘B’ already stands for blue, so ‘K’ stands for black, hence CMYK) to reduce the amount of ink needed to make black. This also accommodates for the fact that most CMY inks alone can’t make a very deep, rich black.
Due to the differences between transmissive light and reflected light, as well as the fact that inks don’t necessarily match the theoretical cyan, magenta, and yellow colors needed to reproduce a wide range of colors, CMYK processes generally reproduce a much smaller subset of the colors of a typical RGB display. Color management attempts to mitigate this discrepancy, but it is far from perfect. Inkjet manufacturers and printing ink manufacturers have attempted to expand the gamut of possible colors by adding additional colors to the mix.
If colour is critical, please provide a proof for us to match to, and ask for a hard copy proof. We ALWAYS recommend that clients provide proofs to match if colour is critical, and recommend that clients with colour critical job ask for a hard copy proof.
In some 20 years of pre-press we have never relied on what is shown on screen. We have owned $40,000 calibrated monitors in the past and still have no faith in anything but a CMYK breakdown for matching specific colours. Even then you need to have an accurate printed reference to match to as well — such as a Pantone Process book. There are many Pantone Books available. Consider purchasing a ‘PANTONE color bridge coated’ book.
Digital prints are good and getting better all the time, but when it comes to calibration ‘out of the box’ with no operator intervention we have yet to see or use one that is truly accurate. Tweaking by an operator always seems to be needed. ‘I know my Epson printer at home tends to do this . . . ’ — enough said!
Skin-tones are the worst. A few percentage points of one ink in any direction can throw the cast off altogether.